1. Field
The disclosure relates to a metal ring and a manufacturing method for the metal ring, especially to a metal ring in a transmission belt of a belt-type continuously variable transmission, and a manufacturing method for the metal ring.
2. Description of Related Art
A steel belt-type continuously variable transmission (CVT), in which an input side pulley and an output side pulley are connected to each other by a steel-made transmission belt, is used in an automobile and so on. A transmission belt in the steel belt-type CVT has a structure in which a number of elements are lined up without space between them and attached to a metal ring laminate that is made from a plurality of thin-sheet metal rings laminated in a nested state. Tension of the metal ring laminate presses the elements against input-side and output-side pulleys, thereby transmitting power from the input-side pulley to the output-side pulley.
In order to ensure friction force between the elements and the input-side and output-side pulleys, high tension is applied to each of the metal rings that structure the metal ring laminate. Therefore, maraging steel, which is precipitation strengthened ultra high strength steel, is used for the metal rings. Also, repeated bending stress is also applied to the metal rings in a high tension state. Therefore, for the purpose of improving fatigue strength, nitriding is performed for giving compressive residual stress to surfaces of the metal rings.
In normal nitriding, uniform nitrided layers are formed in main surfaces (inner and outer peripheral surfaces) and end surfaces of the metal ring. Alternatively, the nitrided layers formed in the end surfaces (nitrided layers on end parts) become thicker than the nitrided layers formed in the main surfaces (nitrided layers in the main surface parts). When thicknesses of the nitrided layers in the end parts become large, residual tensile stress in non-nitrided parts on inner sides of the nitrided layers of the end parts becomes large, and there is thus a possibility that fatigue fracture starting from the end parts could happen easily.
In order to remove such a possibility, International Unexamined Patent Application No. 2011/135624 discloses a metal ring in which a thickness of a nitrided layer in an end part is reduced by forming a nitriding inhibition film on an end surface before nitriding. Because the thickness of the nitrided layer of the end part is reduced, fatigue fracture starting from the end part is restrained.